Using an electron beam for testing integrated circuits is described in an article by E. Menzel et al., entitled "Electron Beam Testing Techniques," published in Microelectronics Engineering, Volume 16, pp. 3-14, March 1992. The article discloses how to direct an electron beam onto an energized circuit, which produces secondary electrons that can be detected when they are emitted by the material of which the circuit is composed. A suitable algorithm is then used to determine a surface voltage of the circuit.
The principle by which an electron beam 2 of energy E.sub.0 interacts with a substrate 4 is described with reference to FIGS. 1 and 2. The energy E.sub.0 enables the beam to penetrate the surface of substrate 4 to a given depth. The interaction between the electrons and the material causes an emission of photons 6 and an emission of secondary electrons 8. The spectrum of the electrons emitted (see FIG. 2) includes, in addition to an elastic peak 10 at the value of the incident energy E.sub.0, Auger electrons 12 and secondary electrons 14.
In practice, a substrate 4 is tested with an electron beam 2 using the apparatus or scanning electron microscope shown in FIG. 3. An electron gun 16 generates electron beam 2 that passes successively through electronic lenses 18 and beam deflection means 20. The secondary electrons 8 are detected by a detector 22, the signal of which is shown on display means 24 after processing by suitable processing means 26. Synchronization means 27 synchronizes the signals applied to deflection coils 20 and display means 24.
This type of apparatus displays differences in potentials on the surface of substrate 4. Qualitative measurements of differences in potentials can be made as described below with reference to FIG. 4. The surface of substrate 4 includes conducting tracks 28, 30, 32 that are at different potentials. For example, track 30 is at a potential of 5 volts while tracks 28 and 32 are maintained at a potential of 0 volts. A primary electron beam 2 is focused on track 30 which causes emission of secondary beams 8. A primary electron beam 32 is focused on track 28 and produces a beam of secondary electrons 38.
The potential distribution at the surface of substrate 4 determines the distribution of equipotential surfaces above this substrate 4. In FIG. 4, equipotential surfaces with values of 1 volt, 2 volts, 3 volts, 4 volts and 5 volts are shown. The shape of these equipotentials shows that only electrons with sufficient energy can escape towards the detector corresponding to track 30. A potential barrier, which is shown as reference 34, is established at a value determined by the value of the potential at the surface of substrate 4.
In the example given above, the potential barrier 34 is at approximately 3.5 volts when track 30 has a potential of 5 volts. Consequently, only electrons of energy greater than 1.5 eV can join secondary beam 8 and be detected. Track 28 is maintained at a potential of 0 volts, and no barrier effect is created. All secondary electrons, irrespective of their energy, can join secondary beam 38.
Various forms of secondary electron spectra are possible. In the case of track 30, which has a potential of 5 volts, the low energy section of the spectrum has a break at approximately 1.5 eV (see FIG. 5A). In track 28, which has a potential of 0 volts, no break appears (see FIG. 5B). If the development in the intensity of the signal of detector 22 is plotted against the potential V in volts of the track being tested, a curve illustrated in FIG. 5C is obtained for a given electrical environment. The lower the potential of the track, the more powerful the signal detected.
According to one variation of the apparatus shown in FIG. 6, an electrostatic filter is placed in the path of the secondary electrons and is increased to a potential V.sub.f. The track is at a potential V.sub.p and the electrons need sufficient energy to pass not only through the extraction barrier (potential V.sub.ext), but also the electrostatic barrier (potential V.sub.f). Once the electrons have passed through the filter, they are accelerated in the direction of the detector 8. The detector 8 supplies a signal of intensity I that is based upon the difference V.sub.p -V.sub.f.
It is not feasible to measure continuous voltage levels with such a conventional electron beam apparatus. For example, it is impossible to measure DC voltages on the power supply rail of a circuit. Only measurements of variations in surface potentials between one point and another on the surface of a substrate or integrated circuit are measured. In addition, only variations in potential at a given point on the surface from one moment to the next are measured.
Movement of an electron beam from one point to another is electronically controlled using a suitable program. Furthermore, the apparatus only uses a single measuring channel. Measuring between two points on the surface of a circuit involves three main steps: 1) a measurement is taken at a first point using an apparatus with a single measuring channel, and a corresponding waveform is observed while scales on the order of a few nanoseconds per division are commonly used for this waveform; 2) the first point is slowly moved to a second measuring point; and 3) a second measurement is taken when the beam is in position on the second point. A measurement of the relative phase variations between the two points is thus obtained.
To measure DC voltage levels, a mechanical probe is brought into contact with the conducting track. Consequently, there is a risk in this technique of causing damage to the circuit. Measurements obtained with a mechanical probe (for DC voltage levels) are combined with measurements made using an electron beam (for measuring differences in voltage on the surface of a circuit) to test circuits housed in a unit. Therefore, a problem arises to find a method and an apparatus that can test DC voltage levels in a circuit that avoids physical contact with the circuit.